1. Field of the Invention
The present invention relates to an optical beam scanning device used for a laser printer, a digital copying machine or the like, and particularly to a pre-deflection optical system for guiding a light beam from a light source to a rotary deflector which deflects an incident light flux and scans it in a specified scanning direction.
2. Description of the Related Art
Hitherto, there is known a light beam scanning device which uses a rotary deflector such as a polygon mirror and scans a light flux from a light source in a main scanning direction. In such a conventional light beam scanning device, as a pre-deflection optical system to shape a divergent light beam from a light source into a light flux having a cross-sectional shape long in the main scanning direction, there are often adopted a lens to weaken the degree of divergence of the divergent light beam from the light source or to convert it into a parallel light beam or a converging light beam, and a lens to give a positive power to the light flux the degree of divergence of which has been weakened or which has been converted into the parallel light beam or the converging light beam.
However, in a case where a multi-beam optical system using divergent light beams from plural light sources is adopted in the light beam scanning device, when only the structure of the conventional light beam scanning device is used, it is difficult to adjust all conditions, such as, in the rotation axis direction of the rotary deflector, (1) the pitch of the light fluxes from the plural light sources, (2) focal positions and (3) beam diameters of the light fluxes, to be appropriate.